DESCRIPTION (Applicant's Description): The primary objective of this proposal is to complete a 3 component career development plan that will result in the applicant's acquiring the necessary skills and experience to develop an independent translational research program investigating the genetic mechanisms responsible for malignant transformation in children. The 3 components of this plan are: 1) a one-year co-mentorship with two senior cancer researcher scientists who will provide him with the guidance and expertise required to complete the academic and research portion of his training: 2) a didactic program that focuses on specific areas of cancer biology; and 3) a transition segment from colleague/mentor to independent research scientist and collaborator during the independent research portion of his proposal. The principal research objectives of this proposal are to investigate the molecular mechanisms and biologic consequences of in vivo somatic genetic events responsible for pediatric malignancies. The hypothesis of the applicants is that somatic mutational events in children with cancer will occur at a higher frequency and with a unique mutational spectra compared to a normal population. The specific aims which will test the proposed hypotheses are: 1) to determine the frequency and mutational spectra of background and t h erapy induced in vivo somatic mutations in children with specific malignancies (acute lymphocytic leukemia [ALL], Hodgkin's disease, neuroblastoma, and sarcomas) and where possible, to correlate these molecular events with subsequent diseases: 2) to determine if "illegitimate" V(D)J recombinase mediated mutations occur at a known cancer gene, p53; and 3) to isolate and identify T-lymphocyte imitator phenotype clones from children with relapsed ALL who have an extremely high frequency of background somatic mutations. The frequency of somatic mutational events will be determined by the hprt T-cell cloning assay. Mutational spectra of hprt and p53 mutations will be determined by a variety of methods including multiplex PCR, RT-PCR, IPCR, Southern blotting and DNA sequencing. Clonality of hypermutable clones will be determined by RFLP analysis of TCR gamma and CDR3 region DNA sequence analysis of TCR-beta. These studies will provide the first data on the frequency and mutational spectrum of in vivo spontaneous or treatment induced somatic mutations in children with specific malignancies. The investigators will also begin to determine the potential mutagenic effects associated [with] background "illegitimate" V(D)J recombinase mediated events in a known cancer gene, p53. In addition, the isolation of mutator phenotype clones will allow for future m e chanistic studies characterizing the genetic and biologic defect(s) associated with leukemogenesis in children.